1. Field of the Invention
The present invention relates to an image forming apparatus capable of detecting original-images density such as a copying machine. More particularly, the invention relates to an image forming apparatus capable of detecting the original-image density by detecting the quantity of light reflected by an original which is illuminated by an exposure light source.
2. Description of the Prior Art
In recent years, in order to obtain a good image, an image forming apparatus such as a copying machine employs an automatic exposure unit for automatically controlling an amount of light radiated on a document from an exposure light source in correspondence to the density of an original image.
Such an automatic exposure unit detects an original-image density by some means, and changes an exposure quantity of light and the like in correspondence thereto, thereby obtaining a proper image. Examples of a conventional original-image density detecting device used in the above unit are shown in FIFS. 1A and 1B and FIGS. 2A and 2B.
In the device shown in FIGS. 1A and 1B, light emitted from exposure light source 1 is radiated on original 2, and reflected light beam 3 from original 2 is converged on sensor 8 formed by a photoelectric conversion element such as a diode or a phototransistor by SELFOC lens (a trade name of a self focusing lens) 7 for detection. Thus, the intensity of reflected light is detected to detect the image density of original 2 (Japanese Patent Disclosure (Kokai) No. 57-210370). Lens 7 is provided near SELFOC lens 6 for guiding the reflected light beam from original 2 on photosensitive body 4, and its dimension in the widthwise direction of the photosensitive body is increased as long as possible to enlarge a converging area, thereby improving detection accuracy of image density. That is, as shown in FIG. 1B, light-receiving surface 9 on an upper portion of lens 7 is elongated to focus reflected light within a range as wide as possible to sensor 8.
However, in this case, although the detection accuracy is improved, such a structure is so complicated as to make manufacture difficult, resulting in an expensive apparatus.
In the device shown in FIGS. 2A and 2B, box member 11 is provided near SELFOC lens 6, curved reflecting plate 12 is provided on a bottom portion of box member 11, and sensor 13 is provided at a focal point of plate 12. Light beam from exposure light source 1 reflected by original 2 and reaching around lens 6 is further reflected 12 and converged on sensor 13. Thus, the quantity or reflected light is detected to detect the image density of original 2 (Japanese Patent Disclosure (KOKAI) No. 58-214142 and Japanese Utility Model Disclosure (KOKAI) No. 58-62357). Also in this device, the dimension of an opening at an upper portion of box member 11 is increased along the widthwise direction of a photosensitive body to improve detection accuracy.
However, in this case, high accuracy is required for a reflecting surface of plate 12 to focus the reflected light of wide range to sensor 13, resulting in cumbersome manufacture and an expensive apparatus as described above.
Thus, the above conventional apparatuses are both expensive to prevent realization of an inexpensive copying machine, make it difficult to apply an automatic exposure unit to a home copying machine, and prevent improvement in image quality.